Membrane-associated proteins and protein complexes account for −30% or more of the cellular proteins. Membrane proteins are held within a bilayer structure. The basic membrane bilayer construct consists of two opposing layers of amphiphilic molecules know as phospholipids; each molecule has a hydrophilic moiety, i.e., a polar phosphate group/derivative, and a hydrophobic moiety, i.e., a long hydrocarbon chain. These molecules self-assemble in a biological (largely aqueous) environment according to thermodynamics associated with water exclusion or hydrophobic association.
Some membrane associated proteins are also biocatalysts, i.e. biological compounds able to affect the rate of a chemical reaction, such as transformations involving organic compounds.
In particular, certain membrane proteins play a key role in the transport of energy across cellular boundaries. Recently, several membrane proteins have been identified as attractive candidates for enabling the conversion of biomass and/or solar energy to renewable fuels. Among these enzymes are hydrogenases, which can function to catalyze the production of molecular hydrogen from protons.
Constructions of ex vivo and/or in vitro biocatalytic systems including hydrogenase is often challenging in view of the need to reproduce the conditions necessary to provide the enzyme in an active form and in view of the instability to oxygen and sensitivity to byproducts that characterize several of those biocatalysts.